An endoscope system having a solid-state imaging device consists mainly of an endoscope such as an electronic endoscope, a processor, a light source unit, and a monitor. In the endoscope system, the insertion unit of the endoscope is inserted into a body cavity, and illumination light emanating from the light source unit is irradiated to an object over a light guide lying through the endoscope. The solid-state imaging device incorporated in the distal part of the endoscope photoelectrically converts the light to produce a video signal. The processor processes the signal and displays an image on the monitor according to the signal.
Talking of the endoscope system, a field-sequential endoscope system like the one disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1-221135 is known as a modality enabling observation under ordinary light by utilizing illumination light of wavelengths falling within the visible spectrum. In the endoscope system, as described in Japanese Unexamined Patent Application Publication No. 9-70384, an endoscope designed for fluorescence diagnosis is often employed in order to discover an early-stage carcinoma or the like. Specifically, excitation light is irradiated to a living tissue, and light stemming from fluorescence exhibited by the living tissue is observed in order to discover an early-stage carcinoma or the like.
An imaging device included in such a fluorescence diagnosis endoscope system is requested to offer so high sensitivity as to enable observation of feeble light stemming from fluorescence. For this reason, a pickup tube is often employed. Japanese Unexamined Patent Application Publication No. 5-252450 has disclosed a technology of controlling a drain voltage occurring due to overflow in a solid-state imaging device according to an output signal of the solid-state imaging device. The technology thus enables visualization of a region whose image cannot be corrected by controlling an amount of light using an iris diaphragm.
Problems to be Solved by the Invention
In the foregoing endoscope system, various types of endoscopes are switched for use according to a region to be assessed or a method of assessment. For example, an endoscope dedicated to examination of the bronchi is thinner than an endoscope dedicated to examination of the large intestine.
The diameter of an endoscope affects the number of optical fibers constituting a light guide lying through the endoscope, and brings about a difference in the amount of irradiated light. Moreover, an f-number varies depending on the purpose of use of an endoscope. In particular, when an endoscope having a large f-number set therefore is used to observe an object located at a far point, the amount of light is so small that a view image is dark.
This causes a range, within which a proper amount of light necessary for picking up image data is collected, to greatly vary depending on a type of endoscope. On the other hand, as mentioned above, the endoscope system is usable not only for observation under ordinary light but also for observation under special light such as light stemming from fluorescence intended to assess a lesion. For the observation under light stemming from fluorescence, very feeble light stemming from auto-fluorescence must be collected. Therefore, a solid-state imaging device to be incorporated in the distal part of an endoscope is requested to offer much higher sensitivity than a solid-state imaging device designed for observation under ordinary light.
In general, when the endoscope system is used to observe an object that makes quick motion or to produce a still image, the solid-state imaging device is driven using an electronic shutter. In this case, the amount of irradiated light is increased in order to optimize an exposure value. However, when an iris diaphragm is fully opened in order to adjust the amount of irradiated light, if the electronic shutter is activated, the exposure value becomes insufficient. This results in a dark image. Automatic gain control (AGC) may be utilized to compensate the insufficient exposure value. However, this poses a problem in that a noise is intensified.
An object of the present invention is to provide an endoscope system capable of producing a view image of proper brightness irrespective of a type of endoscope. Specifically, the sensitivity of a solid-state imaging device is controlled depending on the type of endoscope, that is, the diameter of an insertion unit of an endoscope, an f-number set for an endoscope, or whether an endoscope is designed for observation under ordinary light or observation under special light such as light stemming from fluorescence.
Another object of the present invention is to provide an endoscope system capable of offering a proper exposure value by controlling the sensitivity of a solid-state imaging device according to movement information concerning the light source, whether an amount of light supplied from a light source is insufficient or not.
Still another object of the present invention is to provide an endoscope system capable of producing a view image less affected by a noise by controlling the sensitivity of a solid-state imaging device according to the driven state of the solid-state imaging device.